1 //! See Rustc Dev Guide chapters on [trait-resolution] and [trait-specialization] for more info on
4 //! [trait-resolution]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
5 //! [trait-specialization]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html
7 use crate::infer::{CombinedSnapshot, InferOk, TyCtxtInferExt};
8 use crate::traits::query::evaluate_obligation::InferCtxtExt;
9 use crate::traits::select::IntercrateAmbiguityCause;
10 use crate::traits::util::impl_trait_ref_and_oblig;
11 use crate::traits::SkipLeakCheck;
13 self, FulfillmentContext, Normalized, Obligation, ObligationCause, PredicateObligation,
14 PredicateObligations, SelectionContext,
16 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
17 use rustc_middle::ty::fast_reject::{self, SimplifyParams, StripReferences};
18 use rustc_middle::ty::fold::TypeFoldable;
19 use rustc_middle::ty::subst::Subst;
20 use rustc_middle::ty::{self, Ty, TyCtxt};
21 use rustc_span::symbol::sym;
22 use rustc_span::DUMMY_SP;
25 /// Whether we do the orphan check relative to this crate or
26 /// to some remote crate.
27 #[derive(Copy, Clone, Debug)]
33 #[derive(Debug, Copy, Clone)]
39 pub struct OverlapResult<'tcx> {
40 pub impl_header: ty::ImplHeader<'tcx>,
41 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
43 /// `true` if the overlap might've been permitted before the shift
45 pub involves_placeholder: bool,
48 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
50 "this behavior recently changed as a result of a bug fix; \
51 see rust-lang/rust#56105 for details",
55 /// If there are types that satisfy both impls, invokes `on_overlap`
56 /// with a suitably-freshened `ImplHeader` with those types
57 /// substituted. Otherwise, invokes `no_overlap`.
58 #[instrument(skip(tcx, skip_leak_check, on_overlap, no_overlap), level = "debug")]
59 pub fn overlapping_impls<F1, F2, R>(
63 skip_leak_check: SkipLeakCheck,
68 F1: FnOnce(OverlapResult<'_>) -> R,
71 // Before doing expensive operations like entering an inference context, do
72 // a quick check via fast_reject to tell if the impl headers could possibly
74 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
75 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
77 // Check if any of the input types definitely do not unify.
79 impl1_ref.iter().flat_map(|tref| tref.substs.types()),
80 impl2_ref.iter().flat_map(|tref| tref.substs.types()),
83 let t1 = fast_reject::simplify_type(tcx, ty1, SimplifyParams::No, StripReferences::No);
84 let t2 = fast_reject::simplify_type(tcx, ty2, SimplifyParams::No, StripReferences::No);
86 if let (Some(t1), Some(t2)) = (t1, t2) {
87 // Simplified successfully
94 // Some types involved are definitely different, so the impls couldn't possibly overlap.
95 debug!("overlapping_impls: fast_reject early-exit");
99 let overlaps = tcx.infer_ctxt().enter(|infcx| {
100 let selcx = &mut SelectionContext::intercrate(&infcx);
101 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some()
108 // In the case where we detect an error, run the check again, but
109 // this time tracking intercrate ambuiguity causes for better
110 // diagnostics. (These take time and can lead to false errors.)
111 tcx.infer_ctxt().enter(|infcx| {
112 let selcx = &mut SelectionContext::intercrate(&infcx);
113 selcx.enable_tracking_intercrate_ambiguity_causes();
114 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
118 fn with_fresh_ty_vars<'cx, 'tcx>(
119 selcx: &mut SelectionContext<'cx, 'tcx>,
120 param_env: ty::ParamEnv<'tcx>,
122 ) -> ty::ImplHeader<'tcx> {
123 let tcx = selcx.tcx();
124 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
126 let header = ty::ImplHeader {
128 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
129 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
130 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
133 let Normalized { value: mut header, obligations } =
134 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
136 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
140 /// What kind of overlap check are we doing -- this exists just for testing and feature-gating
142 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
144 /// The 1.0 rules (either types fail to unify, or where clauses are not implemented for crate-local types)
146 /// Feature-gated test: Stable, *or* there is an explicit negative impl that rules out one of the where-clauses.
148 /// Just check for negative impls, not for "where clause not implemented": used for testing.
153 fn use_negative_impl(&self) -> bool {
154 *self == OverlapMode::Strict || *self == OverlapMode::WithNegative
157 fn use_implicit_negative(&self) -> bool {
158 *self == OverlapMode::Stable || *self == OverlapMode::WithNegative
162 fn overlap_mode<'tcx>(tcx: TyCtxt<'tcx>, impl1_def_id: DefId, impl2_def_id: DefId) -> OverlapMode {
163 if tcx.has_attr(impl1_def_id, sym::rustc_strict_coherence)
164 != tcx.has_attr(impl2_def_id, sym::rustc_strict_coherence)
166 bug!("Use strict coherence on both impls",);
169 if tcx.has_attr(impl1_def_id, sym::rustc_with_negative_coherence)
170 != tcx.has_attr(impl2_def_id, sym::rustc_with_negative_coherence)
172 bug!("Use with negative coherence on both impls",);
175 if tcx.has_attr(impl1_def_id, sym::rustc_strict_coherence) {
177 } else if tcx.has_attr(impl1_def_id, sym::rustc_with_negative_coherence) {
178 OverlapMode::WithNegative
184 /// Can both impl `a` and impl `b` be satisfied by a common type (including
185 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
186 fn overlap<'cx, 'tcx>(
187 selcx: &mut SelectionContext<'cx, 'tcx>,
188 skip_leak_check: SkipLeakCheck,
191 ) -> Option<OverlapResult<'tcx>> {
192 debug!("overlap(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
194 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
195 overlap_within_probe(selcx, skip_leak_check, impl1_def_id, impl2_def_id, snapshot)
199 fn overlap_within_probe<'cx, 'tcx>(
200 selcx: &mut SelectionContext<'cx, 'tcx>,
201 skip_leak_check: SkipLeakCheck,
204 snapshot: &CombinedSnapshot<'_, 'tcx>,
205 ) -> Option<OverlapResult<'tcx>> {
206 let infcx = selcx.infcx();
209 let overlap_mode = overlap_mode(tcx, impl1_def_id, impl2_def_id);
211 if overlap_mode.use_negative_impl() {
212 if negative_impl(selcx, impl1_def_id, impl2_def_id)
213 || negative_impl(selcx, impl2_def_id, impl1_def_id)
219 // For the purposes of this check, we don't bring any placeholder
220 // types into scope; instead, we replace the generic types with
221 // fresh type variables, and hence we do our evaluations in an
222 // empty environment.
223 let param_env = ty::ParamEnv::empty();
225 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
226 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
228 debug!("overlap: impl1_header={:?}", impl1_header);
229 debug!("overlap: impl2_header={:?}", impl2_header);
231 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
232 debug!("overlap: unification check succeeded");
234 if overlap_mode.use_implicit_negative() {
235 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
240 if !skip_leak_check.is_yes() {
241 if infcx.leak_check(true, snapshot).is_err() {
242 debug!("overlap: leak check failed");
247 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
248 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
250 let involves_placeholder =
251 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
253 let impl_header = selcx.infcx().resolve_vars_if_possible(impl1_header);
254 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
257 fn equate_impl_headers<'cx, 'tcx>(
258 selcx: &mut SelectionContext<'cx, 'tcx>,
259 impl1_header: &ty::ImplHeader<'tcx>,
260 impl2_header: &ty::ImplHeader<'tcx>,
261 ) -> Option<PredicateObligations<'tcx>> {
262 // Do `a` and `b` unify? If not, no overlap.
265 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
266 .eq_impl_headers(impl1_header, impl2_header)
267 .map(|infer_ok| infer_ok.obligations)
271 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
272 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
273 fn implicit_negative<'cx, 'tcx>(
274 selcx: &mut SelectionContext<'cx, 'tcx>,
275 param_env: ty::ParamEnv<'tcx>,
276 impl1_header: &ty::ImplHeader<'tcx>,
277 impl2_header: ty::ImplHeader<'tcx>,
278 obligations: PredicateObligations<'tcx>,
280 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
283 // For example, given these two impl headers:
285 // `impl<'a> From<&'a str> for Box<dyn Error>`
286 // `impl<E> From<E> for Box<dyn Error> where E: Error`
290 // `Box<dyn Error>: From<&'?a str>`
291 // `Box<dyn Error>: From<?E>`
293 // After equating the two headers:
295 // `Box<dyn Error> = Box<dyn Error>`
296 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
298 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
299 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
300 // at some point an impl for `&'?a str: Error` could be added.
301 let infcx = selcx.infcx();
303 let opt_failing_obligation = impl1_header
307 .chain(impl2_header.predicates)
308 .map(|p| infcx.resolve_vars_if_possible(p))
309 .map(|p| Obligation {
310 cause: ObligationCause::dummy(),
317 loose_check(selcx, o) || tcx.features().negative_impls && negative_impl_exists(selcx, o)
319 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
320 // to the canonical trait query form, `infcx.predicate_may_hold`, once
321 // the new system supports intercrate mode (which coherence needs).
323 if let Some(failing_obligation) = opt_failing_obligation {
324 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
331 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
332 /// where-clauses) If so, return true, they are disjoint and false otherwise.
333 fn negative_impl<'cx, 'tcx>(
334 selcx: &mut SelectionContext<'cx, 'tcx>,
338 let tcx = selcx.infcx().tcx;
340 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
341 let impl1_env = tcx.param_env(impl1_def_id);
342 let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();
344 // Create an infcx, taking the predicates of impl1 as assumptions:
345 tcx.infer_ctxt().enter(|infcx| {
346 // Normalize the trait reference. The WF rules ought to ensure
347 // that this always succeeds.
348 let impl1_trait_ref = match traits::fully_normalize(
350 FulfillmentContext::new(),
351 ObligationCause::dummy(),
355 Ok(impl1_trait_ref) => impl1_trait_ref,
357 bug!("failed to fully normalize {:?}: {:?}", impl1_trait_ref, err);
361 // Attempt to prove that impl2 applies, given all of the above.
362 let selcx = &mut SelectionContext::new(&infcx);
363 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
364 let (impl2_trait_ref, obligations) =
365 impl_trait_ref_and_oblig(selcx, impl1_env, impl2_def_id, impl2_substs);
367 // do the impls unify? If not, not disjoint.
368 let more_obligations = match infcx
369 .at(&ObligationCause::dummy(), impl1_env)
370 .eq(impl1_trait_ref, impl2_trait_ref)
372 Ok(InferOk { obligations, .. }) => obligations,
375 "explicit_disjoint: {:?} does not unify with {:?}",
376 impl1_trait_ref, impl2_trait_ref
382 let opt_failing_obligation = obligations
384 .chain(more_obligations)
385 .find(|o| negative_impl_exists(selcx, o));
387 if let Some(failing_obligation) = opt_failing_obligation {
388 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
396 fn loose_check<'cx, 'tcx>(
397 selcx: &mut SelectionContext<'cx, 'tcx>,
398 o: &PredicateObligation<'tcx>,
400 !selcx.predicate_may_hold_fatal(o)
403 fn negative_impl_exists<'cx, 'tcx>(
404 selcx: &SelectionContext<'cx, 'tcx>,
405 o: &PredicateObligation<'tcx>,
407 let infcx = selcx.infcx();
412 // FIXME This isn't quite correct, regions should be included
413 selcx.infcx().predicate_must_hold_modulo_regions(o)
418 pub fn trait_ref_is_knowable<'tcx>(
420 trait_ref: ty::TraitRef<'tcx>,
421 ) -> Option<Conflict> {
422 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
423 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
424 // A downstream or cousin crate is allowed to implement some
425 // substitution of this trait-ref.
426 return Some(Conflict::Downstream);
429 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
430 // This is a local or fundamental trait, so future-compatibility
431 // is no concern. We know that downstream/cousin crates are not
432 // allowed to implement a substitution of this trait ref, which
433 // means impls could only come from dependencies of this crate,
434 // which we already know about.
438 // This is a remote non-fundamental trait, so if another crate
439 // can be the "final owner" of a substitution of this trait-ref,
440 // they are allowed to implement it future-compatibly.
442 // However, if we are a final owner, then nobody else can be,
443 // and if we are an intermediate owner, then we don't care
444 // about future-compatibility, which means that we're OK if
446 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
447 debug!("trait_ref_is_knowable: orphan check passed");
450 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
451 Some(Conflict::Upstream)
455 pub fn trait_ref_is_local_or_fundamental<'tcx>(
457 trait_ref: ty::TraitRef<'tcx>,
459 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
462 pub enum OrphanCheckErr<'tcx> {
463 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
464 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
467 /// Checks the coherence orphan rules. `impl_def_id` should be the
468 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
469 /// two conditions must be satisfied:
471 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
472 /// 2. Some local type must appear in `Self`.
473 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
474 debug!("orphan_check({:?})", impl_def_id);
476 // We only except this routine to be invoked on implementations
477 // of a trait, not inherent implementations.
478 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
479 debug!("orphan_check: trait_ref={:?}", trait_ref);
481 // If the *trait* is local to the crate, ok.
482 if trait_ref.def_id.is_local() {
483 debug!("trait {:?} is local to current crate", trait_ref.def_id);
487 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
490 /// Checks whether a trait-ref is potentially implementable by a crate.
492 /// The current rule is that a trait-ref orphan checks in a crate C:
494 /// 1. Order the parameters in the trait-ref in subst order - Self first,
495 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
496 /// 2. Of these type parameters, there is at least one type parameter
497 /// in which, walking the type as a tree, you can reach a type local
498 /// to C where all types in-between are fundamental types. Call the
499 /// first such parameter the "local key parameter".
500 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
501 /// going through `Box`, which is fundamental.
502 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
504 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
505 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
506 /// the local type and the type parameter.
507 /// 3. Before this local type, no generic type parameter of the impl must
508 /// be reachable through fundamental types.
509 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
510 /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is
511 /// reachable through the fundamental type `Box`.
512 /// 4. Every type in the local key parameter not known in C, going
513 /// through the parameter's type tree, must appear only as a subtree of
514 /// a type local to C, with only fundamental types between the type
515 /// local to C and the local key parameter.
516 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
517 /// is bad, because the only local type with `T` as a subtree is
518 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
519 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
520 /// the second occurrence of `T` is not a subtree of *any* local type.
521 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
522 /// `LocalType<Vec<T>>`, which is local and has no types between it and
523 /// the type parameter.
525 /// The orphan rules actually serve several different purposes:
527 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
528 /// every type local to one crate is unknown in the other) can't implement
529 /// the same trait-ref. This follows because it can be seen that no such
530 /// type can orphan-check in 2 such crates.
532 /// To check that a local impl follows the orphan rules, we check it in
533 /// InCrate::Local mode, using type parameters for the "generic" types.
535 /// 2. They ground negative reasoning for coherence. If a user wants to
536 /// write both a conditional blanket impl and a specific impl, we need to
537 /// make sure they do not overlap. For example, if we write
539 /// impl<T> IntoIterator for Vec<T>
540 /// impl<T: Iterator> IntoIterator for T
542 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
543 /// We can observe that this holds in the current crate, but we need to make
544 /// sure this will also hold in all unknown crates (both "independent" crates,
545 /// which we need for link-safety, and also child crates, because we don't want
546 /// child crates to get error for impl conflicts in a *dependency*).
548 /// For that, we only allow negative reasoning if, for every assignment to the
549 /// inference variables, every unknown crate would get an orphan error if they
550 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
551 /// mode. That is sound because we already know all the impls from known crates.
553 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
554 /// add "non-blanket" impls without breaking negative reasoning in dependent
555 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
557 /// For that, we only a allow crate to perform negative reasoning on
558 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
560 /// Because we never perform negative reasoning generically (coherence does
561 /// not involve type parameters), this can be interpreted as doing the full
562 /// orphan check (using InCrate::Local mode), substituting non-local known
563 /// types for all inference variables.
565 /// This allows for crates to future-compatibly add impls as long as they
566 /// can't apply to types with a key parameter in a child crate - applying
567 /// the rules, this basically means that every type parameter in the impl
568 /// must appear behind a non-fundamental type (because this is not a
569 /// type-system requirement, crate owners might also go for "semantic
570 /// future-compatibility" involving things such as sealed traits, but
571 /// the above requirement is sufficient, and is necessary in "open world"
574 /// Note that this function is never called for types that have both type
575 /// parameters and inference variables.
576 fn orphan_check_trait_ref<'tcx>(
578 trait_ref: ty::TraitRef<'tcx>,
580 ) -> Result<(), OrphanCheckErr<'tcx>> {
581 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
583 if trait_ref.needs_infer() && trait_ref.needs_subst() {
585 "can't orphan check a trait ref with both params and inference variables {:?}",
590 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
591 // if at least one of the following is true:
593 // - Trait is a local trait
594 // (already checked in orphan_check prior to calling this function)
596 // - At least one of the types T0..=Tn must be a local type.
597 // Let Ti be the first such type.
598 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
600 fn uncover_fundamental_ty<'tcx>(
605 // FIXME: this is currently somewhat overly complicated,
606 // but fixing this requires a more complicated refactor.
607 if !contained_non_local_types(tcx, ty, in_crate).is_empty() {
608 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
610 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
618 let mut non_local_spans = vec![];
619 for (i, input_ty) in trait_ref
622 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
625 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
626 let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate);
627 if non_local_tys.is_empty() {
628 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
630 } else if let ty::Param(_) = input_ty.kind() {
631 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
632 let local_type = trait_ref
635 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
636 .find(|ty| ty_is_local_constructor(ty, in_crate));
638 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
640 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
643 non_local_spans.extend(non_local_tys.into_iter().map(|input_ty| (input_ty, i == 0)));
645 // If we exit above loop, never found a local type.
646 debug!("orphan_check_trait_ref: no local type");
647 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
650 /// Returns a list of relevant non-local types for `ty`.
652 /// This is just `ty` itself unless `ty` is `#[fundamental]`,
653 /// in which case we recursively look into this type.
655 /// If `ty` is local itself, this method returns an empty `Vec`.
659 /// - `u32` is not local, so this returns `[u32]`.
660 /// - for `Foo<u32>`, where `Foo` is a local type, this returns `[]`.
661 /// - `&mut u32` returns `[u32]`, as `&mut` is a fundamental type, similar to `Box`.
662 /// - `Box<Foo<u32>>` returns `[]`, as `Box` is a fundamental type and `Foo` is local.
663 fn contained_non_local_types<'tcx>(
668 if ty_is_local_constructor(ty, in_crate) {
671 match fundamental_ty_inner_tys(tcx, ty) {
673 inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect()
680 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
681 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
682 /// types, returns `None`.
683 fn fundamental_ty_inner_tys<'tcx>(
686 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
687 let (first_ty, rest_tys) = match *ty.kind() {
688 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
689 ty::Adt(def, substs) if def.is_fundamental() => {
690 let mut types = substs.types();
692 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
696 tcx.def_span(def.did),
697 "`#[fundamental]` requires at least one type parameter",
703 Some(first_ty) => (first_ty, types),
709 Some(iter::once(first_ty).chain(rest_tys))
712 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
714 // The type is local to *this* crate - it will not be
715 // local in any other crate.
716 InCrate::Remote => false,
717 InCrate::Local => def_id.is_local(),
721 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
722 debug!("ty_is_local_constructor({:?})", ty);
740 | ty::Projection(..) => false,
742 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
743 InCrate::Local => false,
744 // The inference variable might be unified with a local
745 // type in that remote crate.
746 InCrate::Remote => true,
749 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
750 ty::Foreign(did) => def_id_is_local(did, in_crate),
752 // This merits some explanation.
753 // Normally, opaque types are not involed when performing
754 // coherence checking, since it is illegal to directly
755 // implement a trait on an opaque type. However, we might
756 // end up looking at an opaque type during coherence checking
757 // if an opaque type gets used within another type (e.g. as
758 // a type parameter). This requires us to decide whether or
759 // not an opaque type should be considered 'local' or not.
761 // We choose to treat all opaque types as non-local, even
762 // those that appear within the same crate. This seems
763 // somewhat surprising at first, but makes sense when
764 // you consider that opaque types are supposed to hide
765 // the underlying type *within the same crate*. When an
766 // opaque type is used from outside the module
767 // where it is declared, it should be impossible to observe
768 // anything about it other than the traits that it implements.
770 // The alternative would be to look at the underlying type
771 // to determine whether or not the opaque type itself should
772 // be considered local. However, this could make it a breaking change
773 // to switch the underlying ('defining') type from a local type
774 // to a remote type. This would violate the rule that opaque
775 // types should be completely opaque apart from the traits
776 // that they implement, so we don't use this behavior.
781 // Similar to the `Opaque` case (#83613).
785 ty::Dynamic(ref tt, ..) => {
786 if let Some(principal) = tt.principal() {
787 def_id_is_local(principal.def_id(), in_crate)
793 ty::Error(_) => true,
795 ty::Generator(..) | ty::GeneratorWitness(..) => {
796 bug!("ty_is_local invoked on unexpected type: {:?}", ty)